No matter how well a pathogen survives and proliferates in a host, it must have a dispersal mechanism so that it can get itself or its descendants into other hosts. For external parasites this can be rather easy. Lice and the fungus that causes ring- worm, for example, are readily spread by personal contact. Internal parasites face greater problems. Those that can regularly get onto the skin have the possibility of contact with other susceptible individu- als. Cold viruses and intestinal bacteria may get onto hands or other surfaces and be spread by handshakes or more intimate contact.

Microorganisms in the bloodstream are not likely to be spread in this way. Many can be transmitted only with the help of biting insects or other transport agents (vectors). Malaria is a well-known example. If there are about ten malarial parasites in the dispersal stage (called gametocytes) in each milligram of blood and a mosquito sucks up three milligrams, it will be taking in about thirty gametocytes. The next item on the mosquito’s agenda is to convert this rich blood meal into eggs and get them fertilized and laid in an environment suitable for development. Meanwhile, the sexually produced offspring of the malarial plasmodia have migrated to the mosquito’s salivary glands, where they transform into an infectious stage in the fluid that will be used to inhibit clotting when the mosquito sucks up its next blood

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meal. The mosquito then unwittingly injects the plasmodia into the next victim. An enormous variety of insects and other organisms can serve as vectors of human diseases.

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